[PATCH net-next v3] tcp: enable mid stream window clamp

The TCP_WINDOW_CLAMP socket option is defined in tcp(7) to "Bound the size
of the advertised window to this value."  Window clamping is distributed
across two variables, window_clamp ("Maximal window to advertise" in
tcp.h) and rcv_ssthresh ("Current window clamp").

This patch updates the function where the window clamp is set to also
reduce the current window clamp, rcv_sshthresh, if needed.  With this,
setting the TCP_WINDOW_CLAMP option has the documented effect of limiting
the window.

Signed-off-by: Neil Spring <redacted>
---
v2: - fix email formatting

v3: - address comments by setting rcv_ssthresh based on prior window 

 net/ipv4/tcp.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index f931def6302e..e8b48df73c85 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c

@@ -3338,6 +3338,7 @@ int tcp_set_window_clamp(struct sock *sk, int val)
 	} else {
 		tp->window_clamp = val < SOCK_MIN_RCVBUF / 2 ?
 			SOCK_MIN_RCVBUF / 2 : val;
+		tp->rcv_ssthresh = min(tp->rcv_wnd, tp->window_clamp);
 	}
 	return 0;
 }

-- 
2.30.2

On Wed, Aug 25, 2021 at 2:02 PM Neil Spring [off-list ref] wrote:

The TCP_WINDOW_CLAMP socket option is defined in tcp(7) to "Bound the size
of the advertised window to this value."  Window clamping is distributed
across two variables, window_clamp ("Maximal window to advertise" in
tcp.h) and rcv_ssthresh ("Current window clamp").

This patch updates the function where the window clamp is set to also
reduce the current window clamp, rcv_sshthresh, if needed.  With this,
setting the TCP_WINDOW_CLAMP option has the documented effect of limiting
the window.

This patch looks like a bug-fix so it should be applied to net not net-next?

On Thu, Aug 26, 2021 at 12:11 PM Yuchung Cheng [off-list ref] wrote:

On Wed, Aug 25, 2021 at 2:02 PM Neil Spring [off-list ref] wrote:

quoted

The TCP_WINDOW_CLAMP socket option is defined in tcp(7) to "Bound the size
of the advertised window to this value."  Window clamping is distributed
across two variables, window_clamp ("Maximal window to advertise" in
tcp.h) and rcv_ssthresh ("Current window clamp").

This patch updates the function where the window clamp is set to also
reduce the current window clamp, rcv_sshthresh, if needed.  With this,
setting the TCP_WINDOW_CLAMP option has the documented effect of limiting
the window.

This patch looks like a bug-fix so it should be applied to net not net-next?

It seems TCP_WINDOW_CLAMP never worked in this context, not sure
if any application was expecting it to work.

Note that if we target net tree, we would like a Fixes: tag.

I will give my SOB a bit later in the day, I have to run some errands.

Thanks.

On 8/25/21 2:01 PM, Neil Spring wrote:

The TCP_WINDOW_CLAMP socket option is defined in tcp(7) to "Bound the size
of the advertised window to this value."  Window clamping is distributed
across two variables, window_clamp ("Maximal window to advertise" in
tcp.h) and rcv_ssthresh ("Current window clamp").

This patch updates the function where the window clamp is set to also
reduce the current window clamp, rcv_sshthresh, if needed.  With this,
setting the TCP_WINDOW_CLAMP option has the documented effect of limiting
the window.

Signed-off-by: Neil Spring <redacted>
---
v2: - fix email formatting

v3: - address comments by setting rcv_ssthresh based on prior window 

SGTM, thanks.

Signed-off-by: Eric Dumazet <edumazet@google.com>

Hello:

This patch was applied to netdev/net-next.git (refs/heads/master):

On Wed, 25 Aug 2021 14:01:17 -0700 you wrote:

The TCP_WINDOW_CLAMP socket option is defined in tcp(7) to "Bound the size
of the advertised window to this value."  Window clamping is distributed
across two variables, window_clamp ("Maximal window to advertise" in
tcp.h) and rcv_ssthresh ("Current window clamp").

This patch updates the function where the window clamp is set to also
reduce the current window clamp, rcv_sshthresh, if needed.  With this,
setting the TCP_WINDOW_CLAMP option has the documented effect of limiting
the window.

[...]

Here is the summary with links:
  - [net-next,v3] tcp: enable mid stream window clamp
    https://git.kernel.org/netdev/net-next/c/3aa7857fe1d7

You are awesome, thank you!
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